The invention relates to a method of dynamically modifying a pseudo-random sequence produced by a pseudo-random sequence generator, which sequence is used for scrambling or descrambling information components at least one part of which is provided in the form of a digital signal divided into packets, the scrambling or descrambling being obtained by combining said pseudo-random sequence with said digital signal, said pseudo-random sequence generator being re-initialized at each start of the packet by means of an initialization word.
The invention also relates to a device for scrambling or descrambling information components at least one part of which is transmitted in the form of a digital signal divided into packets, which device comprises a pseudo-random sequence generator which is re-initialized at each start of the packet, a first non-recursive combination means for forming a pseudo-random sequence on the basis of a series of bits present in the pseudo-random sequence generator, a second combination means for combining the pseudo-random sequence with the digital signal to be scrambled or descrambled.
Such a method and a device are particularly used in television transmissions whose reception is reserved, for example to persons who have paid a subscription fee or who comply with particular criteria (transmissions reserved for certain professions).
Numerous methods and devices corresponding to those described above are known. One of these is, for example the standard system defined in the document "Specification of the D2-MAC/packet system", 1985, pp. 141 to 166. The device comprises a pseudo-random sequence generator which uses an initialization word formed at the receiver end by combining a control word which is determined from a plurality transmitted by a transmitter, a current image field number and a magnitude which is representative of the position of the data packet in the field. The pseudo-random sequence generator has two shift registers. Each of them is fed back to itself and a series of bits is derived therefrom so as to be one of the elements used for forming one of said pseudo-random sequences. At the start of each packet the shift registers are re-initialized, i.e. charged by the initialization word. The plurality transmitted by the transmitter is changed every ten seconds. Due to the fact that the number of the current image field and the position of the data packet in the field influence the construction of the initialization word, this word changes approximately two thousand times per second.
One is currently interested in digital television systems in which neither the fields nor the packets are enumerated. When a known scrambling method is used in these systems, for example the method in accordance with the D2-MAC standard, a fraudulous search of the initialization word will be very easy because it changes only every ten seconds and no longer two thousand times per second, while in ten seconds the fraud has the disposal of many samples all of which are generated with the same pseudo-random sequence for effecting a search and discovering the pseudo-random sequences used for scrambling.
An essential criterion in such a system is the impossibility of fraudulously retrieving the initialization word. This impossibility is relative, which means that this cannot be done within a sufficiently short time for the method in question to be valuable in practice. To this end it is possible to diversify the pseudo-random sequence by influencing the message itself in the creation of the sequence.
However, such a procedure involves the risk of error propagation, i.e. in the case of a transmission error in the transmission channel an error is multiplied by carrying out the descrambling operation at the receiver end.
It is an object of the invention to solve this problem.